A highly specific isozyme of cyclic nucleotide phosphodiesterase, PDE-f, is only present in the testis and appears concurrent with sexual maturation. The testis of the rat contains three isozymes of PDE: PDE-c, PDE-e, and PDE-f. PDE-c is the high Km cyclic AMP phosphodiesterase (6.5x10 minus to the 5th power M) while PDE-f is the low Km isozyme (2.5x10 minus to the 6th power M). PDE-f is localized to the nuclear fragment of the seminiferous tubule and is not present in spermatozoa. Studies will be carried out to ascertain whether PDE-f is under pituitary control. If it is, attempts will be made to induce the isozyme by FSH and ICSH. Attempts will also be made to purify the isozyme and characterize it further.